(a) Field of the Invention
The present invention relates to an exhaust gas purification system, and more particularly, to an exhaust gas purification system capable of reducing harmful materials included in exhaust gas of a vehicle by using solid ammonium salt.
(b) Description of the Related Art
In general, as a technology of reducing nitrogen oxide discharged from an internal combustion engine, particularly a diesel engine, a method of reducing concentration by exhaust gas recirculation (EGR), and a selective catalytic reduction (SCR) which reduces the nitrogen oxide into nitrogen and oxygen by allowing the nitrogen oxide to react on a catalyst by using a reducing agent such as ammonia, urea, or hydrocarbon are used.
In a case in which hydrocarbon such as diesel oil is used in the technology of the selective catalytic reduction, there is a merit in that a subsidiary reducing agent supply apparatus is not necessary because fuel of an internal combustion engine or a combustor is used as a reducing agent, however in a case in which oxygen is present in the exhaust gas, there is a drawback in that performance for reducing the nitrogen oxide is low, because the hydrocarbon reacts with the oxygen in advance.
Considering a technology of the selective catalytic reduction which uses liquid urea, which is another technology of the selective catalytic reduction, when liquid urea, which is made by dissolving urea, which is a material that is present in a solid state at a room temperature, in water, is injected to an exhaust pipe of a vehicle, the liquid urea is converted into ammonia by being thermally decomposed at a temperature equal to or greater than about 150° C., and the ammonia produced as described above reduces the nitrogen oxide into harmless nitrogen with the help of the selective catalytic reduction such as vanadium pentoxide V2O5 or zeolite. The technology of the selective catalytic reduction, which uses the liquid urea, has a merit in that a temperature band of a catalytic reaction is wide, durability is excellent, and high nitrogen oxide purification efficiency of about 60 to 80% may be obtained.
However, because the technology of the selective catalytic reduction using liquid urea as illustrated in FIG. 1 uses the liquid urea by mixing with water, there is a drawback in that a storage container in which the liquid urea is stored becomes unnecessarily huge. In addition, the exhaust gas purification system using the liquid urea has a drawback in which subsidiary devices such as a storage container 3 for storing the liquid urea, a pump 5, an injection module 4, a mixer 6, and the like are required. In addition, as the liquid urea remains in the purification system after an engine is stopped, and the remaining liquid urea becomes frozen by an outside environment at a low temperature (equal to or less than −11° C.), an operation of purifying the exhaust gas is not performed, and damage occurs to the entire system.
In order to complement the aforementioned drawback of the liquid urea, a technology using solid urea (Korean Patent No. 10-0999571) is suggested, but because a thermal decomposition temperature of the solid urea is about 140° C. which is high, there is a drawback in that a large amount of electrical energy or exhaust heat energy is necessary, and when the thermal decomposition temperature is not maintained in the reactor or a pipe, the ammonia is coagulated in the pipe or the like. Further as illustrated in FIG. 2, in addition to the ammonia 3, by-products such as isocyanic acid (HNCO) and cyanuric acid are produced during a process of thermally decomposing the solid urea 2, and as the by-products are cooled and coagulated in a solid state in a pipe, a valve, and a nozzle, as illustrated in FIG. 3, after the engine is stopped, various problems such as damage to a valve, blockage of a nozzle and a pipe, deterioration in purification efficiency and the like occur.
In addition, in the technology using the solid urea, because a period of time until the ammonia is produced by the thermal decomposition of the solid urea at the time of starting a vehicle is necessary, there is a problem in that the exhaust gas, which is not purified, is discharged to the atmosphere during the period of time until the ammonia is produced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.